The chemistry behind diapers

My interests strongly go towards chemistry, and I feel obliged to explain how diapers are capable of capturing urine. For those who are interested enough, I'll include all that I know about the actual chemistry instead of what the packages say. I'll divide this article in two obvious parts: cloth and disposable. As disposable ones are most often used, I'll start out with them. None of the following was taken from Wikipedia.

Definitions:
-Polymer: a polymer is a very long molecule made by putting a lot of tiny molecules together, much like a traffic jam is a very long continues strip of individual cars. The individual cars refer to the individual tiny molecules. A single unit of a polymer is called a monomer.
-Molecule: a molecule is a combination of different non-metalic atoms that, put together, have different properties than the atoms they are made of. See it as Lego, if you have 100 different blocks of Lego, you'll be able to build almost an infinite amount of different objects, also when you use more blocks, there'll be more possibilities.
-Polymerization: polymerization is a chemical reaction where small monomer molecules react with each other to form larger (macro-)molecules. This reaction is often started by using free radicals.
-Terry cloth: a special way of weaving cotton (sometimes combined with polyester to give the cloth extra strenght), to give it the ability to absorb vast quantities of water compared to other ways of weaving cotton.
-Flannel: flannel is a weaving pattern which uses wool, cotton, or other synthetic fibres. Often used in clothing.
-Gauze: Gauze is a rather open and loose way of weaving materials. Previously silk was used. But, nowadays gauze finds its purpose as bandages made out of cotton.
-Mole: I mentioned the word "mol" a few times on this post. See it like this: if you have glass marbles, plastic marbles and steel marbles of exactly the same dimension, they will not weigh the same. If you ask for a pound of plastic marbles you'll get WAY more than when you ask for a pound of steel marbles. So we'll say 10 marbles is one mole. If I ask for 10 moles of plastic marbles now, I'll get just as much marbles as when I'd ask for 10 moles of glass marbles. To convert this story into reality: a real mole is equal to 6.022142 x 10^23 molecules/atoms. This is exactly the amount of atoms in 12 grams of Carbon-12. What this means is basically nothing, it's just something to hold onto that C2H3COONa weighs 94u. This means that it takes 94 grams of the stuff before you can genuinely call it one mol. I got this number from the fact that C (carbon) weighs 12u, it's C2 so that's times two. H weighs 1, times 3, another C weighing 12 There's two Os weighing 16 each, and one Na weighing 23. Combined: 12*2+1*3+12*1+16*2+23*1=94u.
-Sodium acrylate vs Sodium polyacrylate: I do not use the term Sodium polyacrylate in this post to simplify it for people who aren't studying chemistry. When Sodium acrylate polymerizes, it obviously changes into Sodium polyacrylate, this is the stuff you can hold in your hands because the molecules are big enough to form small grain-like particles.

For anyone interested in the meaning of any more definitions in this post, just comment or sent me a note, my knowledge is for everyone, and I much enjoy writing for fellow infantilists!

Disposables:
We all probably know what is inside disposable diapers. The main ingredients include: a hydrophobic layer that comes into contact with the skin, cellulose to spread the moisture evenly, absorbent polymer to actually hold onto the liquid, and an outer layer made out of plastic or cloth to prevent the liquid from getting into your clothes.

The absorbing polymer most used today has the chemistry formula "C2H3COONa", its systematic name is Sodium acrylate, and is made by mixing acrylic acid with Sodium hydroxide. This is an acid-base reaction and it goes as followed:
in words: Acrylic acid + Sodium hydroxide ->forms-> Sodium acrylate + water
in formulas: C2H3COOH + NaOH -> C2H3COONa + H2O

We're not there just yet though. The formed Sodium acrylate must first be dried to get it 100% dry until it can be polymerized. Eventually, one sodium acrylate unit can hold onto 300 individual water molecules. This does not mean it can hold onto 300 times its own weight though, we will go into this later. The polymerization process is done by free radicals. When the sodium acrylate has formed a very long chain known as a polymer, it can be mixed with the cellulose of a diaper. the cellulose also has the purpose to make sure the polymer is evenly spread, otherwise the polymer would just collect at the lowest point, which would make the diaper useless.

More about that absorbency though. We're all interested in the absorbing factor of a disposable diaper! Chemistry books will often ask the following question: how much does it take to absorb a liter of water?. So how much does it take to absorb a full liter of water? Let's assume that a single unit of sodium acrylate is really capable of holding upto 300 molecules of water. So we'll get the following equation:
in words: Sodium acrylate + 300 water ->forms-> Sodium acrylate crystal with 300 water.
in formula C2H3COONa + 300H2O -> C2H3COONa ∙ 300 H2O

Let’s get it a bit more scientific here. C2H3COONa weighs 94gr per mol. Water weighs 18gr per mol. From the equation we’ve seen that one mol of Sodium acrylate can hold onto 300 mol of water. One liter of water weighs 1000 gram. In mol: 1000/18 = 55.6 mol of water. It took 94 gram of Sodium Acrylate to hold 300 mol of water, but a liter of water is only 55.6 mol. So to hold onto a liter of water (55.6 mol) we’ll need: 55.6/300 × 96 gram = 17.4 gram. Did you get all of that? You can read it again if you want to! It basically means 17.4 gram of absorbing polymer is enough to capture an entire liter of water, and about equally as much urine. If we take a moment to think about what we just found: the absorbent polymer can hold onto 57 times its own weight! Whenever a pack of diapers says “maximum absorbency: 1500ml”, they basically mean they put about 25 gram of absorbing polymer inside the diaper. What most of us don't know is that most of the polymer inside a diaper generally doesn't get soaked. Most disposable diapers weren't made big enough for the polymer inside it to expand to 57 times its original size. This is the reason some diapers might explode when you wear them in the shower. Other reason include the fact that some of the absorbing polymer is located at the back side of the diaper, the place where pee normally doesn't get to. Yet another reason a diaper will never reach its maximum absorbency is the fact they start leaking too soon. When a diaper leaks, it cannot absorb the urine fast enough, it does not always mean the polymer is already saturated.

Cloth:
[/B]To begin with, cloth diapers are not as easily described as disposable ones. Disposable diapers all have absorbing polymer to function properly, but cloth diapers depend on a completely different way of capturing water. There also isn’t very much to calculate on this one. Most cloth diapers are entirely made out of polymer. Traditionally, cotton is used to manufacture cloth diapers. But, the introduction of more advanced synthetic polymers has made it possible to achieve a far greater water absorbency than with cotton alone. Cotton is still often used for its soft feel, the fact it’s 100% natural, and its rather low price compared to other polymers. The fact cotton is both soft and friendly for our skin makes it the ideal material to make the inside of a cloth diaper of. Within the diaper you’ll find a variety of materials, though.

Before introducing you to these exotic highly-absorbent washable materials, I’ll explain why some materials absorb water and some don’t. Generally speaking of polymers, you can split them in two groups: hydrophobic and hydrophilic. Hydrophobic, as the name implies, will not absorb water at all and certainly doesn’t want it on it. Sport clothing is sometimes made of polyester. Polyester is a widely used hydrophobic material, and the reason it’s used is to not cause sweat accumulation. Polyester will not absorb your sweat like cotton does, it will simply let it through, so it can evaporate! Hydrophilic materials, as the name implies, love water. There are many polymers capable of holding a small amount of water. But, there is a small group that is capable of absorbing multiple times its own weight in water. Cotton can sometimes absorb up to 10 times its own weight in water. You can try it with a towel to see how much your cotton towels will absorb. Ten times its own weight seems nothing compared to the mind-blowing 57 times of our wonder polymer, but it is as good as it gets for cotton. The material most often used for the absorbent inner core of a cloth diaper is called rayon. Rayon is an artificial material, but it is not made of oil like polyester is. Rayon is made out of natural occurring cellulose, and can be seen as the perfect form of cotton. The reason rayon is not used to make towels out of because it’s not one of the strongest materials. Cotton can stretch to a certain length and it retains its woven form even after damaging parts of it, which makes it the ideal material for making towels out of. Placing rayon inside a cloth diaper will not only assure that it will not be damaged by stretching, but it will offer great absorbency to its user.

Cloth diapers are versatile and delicate objects, and because of the forces put onto them, they will often be strengthened by weaving polyester threads and strips along the sides and the bottom. The use of elastics is also very common practice these days. Elastics are called elastomers, the name is a combination of polymer and elastics. Both polyester and the elastics placed into the cloth diaper to make it stronger, more durable and more comfortable, offer no extra absorbency whatsoever.

The waterproof pants worn over a cloth diaper are often made of either PVC or regular rubber. Rubber is traditionally used, but because of its harsh feeling and low elasticity, it has fallen out of fashion as a waterproof pants. For those willing to spend a little bit extra, rubber pants are still available. PVC stands for Polyvinylchloride and is a hydrophobic polymer. This attribute gives it its ability to keep the moisture inside.

That's about it for now. I hope you've very much enjoyed this introduction into the chemistry of diapers. There is much more to write, but what was written in this post is relevent to general knowledge, that's what I want to believe. Any comments are more than welcome, I spent a lot of time on this post and any comment shows appreciation!

Last edited by redtails; 18-Aug-2008 at 16:54.
Reason: updated again as suggestions told me to, thank you SDA

Very nice, I approve My specialty isn't chemistry but engineering, though I have enough chemistry background to follow your analysis. I have always wondered just how much polymer is used in disposables, and I believe you thoroughly answered that question.

Would you happen to know how widely rayon is used in cloth diapers? I have heard of terry, flannel, gauze, and birdseye as the 'major' fabrics used in cloth diapers, and had assumed that these were primarily made of cotton. My assumption was based on the fact that I have purchased all of these materials from various sources and the majority of them were labeled as 100% cotton; very few were a blend and I do not recall rayon as a significant ingredient.

And if you happen to know of any place that sells fitted cloth diapers made with rayon or a rayon blend (probably as a soaker, since it's delicate), would you let me know?

We actually did an awesome experiment with elementary school kids (I'm an elementary education major) where we took diapers apart and seperated the polymers from the filling to determine which brand had more polymers (we did that by place the insides of the diaper in a ziplock bag, blowing into it, closing it and then shaking it for awhile until the polymer bits all gather at the bottom). Then we put all the polymers in a dish and saw how much water it held before they became over saturated and leaked (we added the water very slowly so that it really was over saturated and not just not absorbing fast enough). The kids loved it. My professor gave me a whole bag of diapers as an end of the semester gag gift, too. Unfortunately they're infant size.

Very nice, I approve My specialty isn't chemistry but engineering, though I have enough chemistry background to follow your analysis. I have always wondered just how much polymer is used in disposables, and I believe you thoroughly answered that question.

Would you happen to know how widely rayon is used in cloth diapers? I have heard of terry, flannel, gauze, and birdseye as the 'major' fabrics used in cloth diapers, and had assumed that these were primarily made of cotton. My assumption was based on the fact that I have purchased all of these materials from various sources and the majority of them were labeled as 100% cotton; very few were a blend and I do not recall rayon as a significant ingredient.

And if you happen to know of any place that sells fitted cloth diapers made with rayon or a rayon blend (probably as a soaker, since it's delicate), would you let me know?

Sure I'll answer all of your questions. The following link will guide you to a quite large Belgian company that makes baby and adult cloth diapers that are almost entirely made out of rayon for its good absorbency: Kiddybips. I can probably list several more, but I'm convinced cotton is not always used as it has many downsides to it, like the fact it wears out after a few hundred cycles while rayon doesn't.

I remember studying and experimenting with Sodium Acrylate back in like 8th grade. It was quite interesting takeing a glass of water and adding a small amount of sodium acrylate to it and watching as it dried and turned to a nearly full solid. It's amazing the chemistry behind the diapers we all love.

Just would like to pop in and say I enjoyed your article a lot Tails! Well written, as always, and it really shows how good and educated you are when it comes to Chemistry! I never tried cloth, but this made me understand a little better, how they actually work and what keeps them absorbant.